In this specification we are concerned with axial flux permanent magnet machines. Broadly speaking these have disc- or ring-shaped rotor and stator structures arranged about an axis. Typically the stator comprises a set of coils each parallel to the axis and the rotor bears a set of permanent magnets and is mounted on a bearing so that it can rotate about the axis driven by fields from the stator coils. FIG. 1a shows the general configuration of an axial flux machine with a pair of rotors R1, R2 to either side of a stator S—although a simple structure could omit one of the rotors. As can be seen there is an air gap G between a rotor and a stator and in an axial flux machine the direction of flux through the air gap is substantially axial.
There are various configurations of axial flux permanent magnet machine depending upon the arrangement of north and south poles on the rotors. FIG. 1b illustrates the basic configurations of a Torus NS machine, a Torus NN machine (which has a thicker yoke because the NN pole arrangement requires flux to flow through the thickness of the yoke), and a YASA (Yokeless and Segmented Armature) topology. The illustration of the YASA topology shows cross-sections through two coils, the cross-hatched area showing the windings around each coil. As can be appreciated, dispensing with the stator yoke provides a substantial saving in weight and iron losses, but drawbacks of removing the stator yoke are a) loss of the structural strength to the stator (which the iron provided) even though there is potentially increased need for strength because of the YASA topology which, being a compact design, can result in very large stresses and b) loss of a route for heat to escape from stator coils. To address both issues, i.e. the high torque density of the YASA design and generation of significant quantities of heat, a housing for the stator should provide great strength and rigidity to address torque demands and should also define a chamber which can be supplied with coolant for the machine. It can further be appreciated from FIG. 1b that for efficient operation (minimum losses in the high reluctance air gap) the gap between the rotor and stator should be as small as possible.
The desired features of a housing for the stator assembly of an axial flux permanent magnet machine, especially one having a YASA topology, impose conflicting requirements. Conventional manufacturing techniques are not able to combine the desired features adequately. General background prior art relating to the production of reinforced articles can be found in, for example, EP0 063 403A; US2001/0028128; and US2008/0292858. We have previously described, in WO2012/022974, a clamshell type housing in which shoes of the stator bars on which the stator coils are wound are over moulded into the radial wall of the housing. However in practice it has proved difficult to form such a strong structure whilst at the same time achieving a very small gap between rotor and stator, preferably around 1.5 mm or less. Moreover injection moulding per se has difficulty in fabricating a radial wall with the desired thinness without substantial risk of perforations, weld lines, and similar faults.